I. Technical Field
The present invention relates to an optical information recording medium, recording/reproducing method thereof, and recording/reproducing apparatus capable of being recorded with and reproducing information signals of a high signal quality as a result of irradiating a thin-film formed on a substrate with a high-energy light beam such as a laser.
II. Description of the Related Art
Extensive research and development have taken place into recording media where a thin-film is formed on a transparent substrate. An information signal is then recorded/reproduced as a result of irradiating the thin-film with laser light focused to give a microscopic spot. Materials where metallic elements that have an oxide as a base material are dispersed on a substrate are well-known as write-once recording media. Such materials include, for example, materials formed with a recording thin-film of TeOx (where 0<x<2) that is a mixture of Te and TeO2 base material (for example, refer to Japanese Unexamined Patent Publication S50-46317). It is possible to obtain a substantial change of reflection rate by irradiating this recording medium with a light beam for reproduction.
TeOx recording thin-films are in an amorphous state after forming a film without performing initialization processing such as laser annealing. This means that it is possible to form recording marks on the crystal by laser irradiation. Recording media employing this recording thin-film can only be utilized as write-once recording media. Correction and deletion by overwriting is not possible because the process is irreversible.
With TeOx recording thin-films, a small amount of time is required until the signal becomes saturated after recording, i.e. until crystallization within the recording thin-film due to irradiation with laser light becomes sufficiently advanced. Media using this recording thin-film are therefore not suited to applications requiring a high-speed response, such as, for example, with computer data files where data is recorded on a disc and is then verified after one rotation, without modification. To overcome this, adding of Pd, or Au etc. to the TeOx as a third element has been proposed (for example, refer to Japanese Unexamined Patent Publication S60-203490, Japanese Unexamined Patent Publication S61-68296, and Japanese Unexamined Patent Publication S62-88152).
It is considered that Pd and Au promote crystalline growth of the Te within the TeOx thin-film during irradiation with laser light. Crystal grains of Te and a Te—Pd alloy or a Te—Au alloy can therefore be formed rapidly. The resistance of Pd and Au to oxidation is also high and there is therefore no degradation of the high moisture resistance of the TeOx thin-film.
Further, methods for improving recording density by shortening the wavelength of the laser light, or making the spot diameter of the laser light smaller by increasing the numerical aperture of an objective lens focusing the light are typically employed as fundamental ways of increasing the amount of information that can be handled by a single medium. Multilayer media where a number of information layers are stacked on top of each other have also been implemented in recent years.
Recording media where the composition of recording material is such that Pd, Au are added to the TeOx and the thin-films are improved are also proposed to implement high-density recording and multi-layer recording (for example, refer to PCT WO98/09823 (page 20-23, FIG. 4)). It is also possible to increase recording density by suppressing thermal interference between marks. This is achieved using the optical enhancing effects and cooling resulting from adding a reflective layer (for example, refer to Japanese Unexamined Patent Publication 2002-251778).
In the above high-density recording, and particularly when a violet-blue laser is employed, the recording layer is subjected to a heat load due to laser heating. This can cause damage and can cause the quality of recorded signals to be degraded as a result of increases in noise, etc. A protective layer of a dielectric etc. is therefore provided in order to prevent this. The following are demanded from the performance of the protective layer: (1) high heat resistance, protecting the recording layer from thermal damage; (2) high adhesion to the recording layer, not causing peeling or diffusing even under high-temperatures and high-humidity; (3) a high degree of transparency and an appropriate refractive index, enhancing optical changes of the recording layer; (4) stability to heat, not fluctuating particle diameter or composition distribution even under high temperatures and high humidity. It is extremely important for write-once recording media that storage reliability is high in addition to high-density recording also being possible. For example, it is also possible for the influence of thermal damage that was not apparent directly after recording to manifest itself and cause noise to increase in the case of, for example, installation under high-temperatures or high-humidity even when sufficient signal quality was obtained at the time of recording.
Noise increases when the recording layer becomes damaged as a result of being heated by a laser so as to be subjected to a heat load. It has therefore been effective to provide a layer with high heat-dissipating properties such as metal as a way of preventing degradation of the quality of the recording signal other than providing a protective layer such as a dielectric. It is therefore typical to give the layer a reflective function using a material having an appropriate optical constant, i.e. make the layer a reflective layer. The optical interference effect is then utilized so that optical absorptance of the recording layer is increased, with optical changes in the recording layer then being enhanced and the recording density being improved. A high resistance to heat, adherence to the neighboring layer, and high storage reliability are also demanded of this reflective layer.